The food service industry requires ovens which provide consistent and efficient heating and cooking of food products. To this end, efforts have been made to employ gas-fired radiant heaters having refractory or ceramic heating elements. However, because of inherent limitations of this type of heater construction, efforts to use this type of heater for food preparation have heretofore met with, at best, limited success.
A conventional gas-fired radiant burner or heater uses an unglazed pottery refractory or ceramic material as the radiating element. The material is heated to a high temperature by combustion of a gas/air mixture at the surface of the material.
The radiating element of refractory material is foraminous in nature, but the multitude of holes thus defined by the element seriously compromises its mechanical strength. As a consequence, such refractory radiating elements are easily broken during assembly on a mass production basis, during transportation of the finished heater unit, or by rough handling by a user of the unit who does not take proper care in handling the fragile material.
The fragile nature of the refractory material can also result in breakage as a result of differential thermal expansion. For example, the radiating element can be broken due to internal stress if, during use, it comes in contact with a cold substance such as water.
The manner in which the usual ceramic or refractory radiating elements normally function further detracts from their versatile and convenient use. In a typical radiant burner apparatus using ceramic radiating elements, a suitable number of ceramic plates are provided with an appropriate number of flame holes defined by the plates to provide the desired heat output. Typically, each ceramic plate is about 5.375 inches by 3.75 inches, and about one-half inch thick. In a typical arrangement, each ceramic plate is formed with about 4,000 holes, each of about 0.050 inches in diameter, whereby about 37% to about 45% of the surface area of the plate is open.
In operation, combustible gas is jetted through a nozzle and is mixed with primary combustion air which is drawn in by the jet stream of gas, or is forced in by a powered blower. Ordinarily, the combustible gas/air mixture is introduced into the burner body through a venturi-type mixing tube, and released into the atmosphere through the small holes bored in the ceramic plates fitted to the upper face of the burner body.
Following ignition of the combustible mixture, gas combustion takes place on the surface of the plate. The flame becomes stable as the surface temperature of the ceramic plate is gradually elevated, and the combustion reaction begins to take place in a layer several thousandths of an inch inside the surface of the plate. This layer is held stationary as long as heating from the surface of the plate, and cooling by the gas/air mixture, are balanced. Radiant heat energy is emitted from the surface of the hot plate.
Difficulties can occur when the gas input increases in a manner which results in excessive elevation of the plate surface temperature, resulting in the gas/air mixture starting to burn at a relatively deep point in the flame holes, thus inducing combustion flashback upstream of the outer surface of the plates. Radiant efficiency as low as 50% is the maximum that can be expected to be obtained because of significant heat loss which is caused by the absorption of heat by the gas mixing tube through the back wall of the ceramic plates.
Because of the nature of the ceramic material, and the requisite formation of a number of flame holes, very little versatility is possible relating to the physical configuration of the plates. Cylindrical or other irregular shapes are virtually impossible to form. The relatively small mechanical strength of the ceramic material further complicates handling of the plates during transportation and assembly.
Heretofore, the use of radiant heater assemblies including ceramic plates for preparation of food products has not been successful due to the inevitable clogging of the flame holes in the ceramic plates with grease and other food debris. Moreover, the ceramic plates exhibit a relatively high degree of thermal inertia, and therefore are not suitable for those applications requiring precise temperature control where the application of heat is required in response to rapid temperature variations.
In view of the foregoing, efforts have been made to replace the typical ceramic or refractory radiating elements with metallic plates. However, such efforts have also been unsuccessful principally because of the problems associated with flashback combustion.
Specifically, in a conventional application, the combustible gas/air mixture ignites above the top surface of the burner. Because of the high temperature to which metallic radiating elements are subjected, on the order of 1,700.degree.-1,800.degree. F., interruption of the gas/air mixture for a period on the order of 3-5 seconds, followed by resupply of the mixture, inevitably results in flashback combustion upstream of the outer radiant surface of the metallic plates. It is believed that this flashback takes place because the combustion gas is reignited by contact with the very hot metallic radiating elements, thus resulting in combustion taking place below the surface or otherwise within the heater assembly, instead of on top of the metallic surface.
The present invention seeks to overcome the various deficiencies associated with radiant heater assemblies employing refractory or ceramic radiating elements, and to further address the shortcomings associated with previous arrangements employing metallic radiating elements in gas-fired radiant heaters.